A typical absolute encoder is a motion detector that provides closed-loop position feedback to a motor control system. Absolute encoders may be utilized in motor control systems where it is desirable to know the instantaneous position of a movable device such as a flood gate of a dam, a robotic arm, a crane, a large telescope, a value, and the like.
FIG. 1 is a block diagram of a conventional optical absolute encoder system 100 which includes a light emitter 102, a light detector 106, and a code disk 108 attached to a shaft 110 capable of rotating. Specifically, light emitter 102 outputs collimated light 104 towards detector 106. However, code disk 108 is located between light emitter 102 and detector 106 such that it interrupts the light 104 received by detector 106. FIG. 2 is a top view of a conventional code disk 108 which includes an exemplary pattern of uniquely coded tracks wherein each unique bit combination received by detector 106 represents a particular rotational position of code disk 108.
For instance, code disk 108 of FIG. 2 has eight tracks and as such, is understood to have an 8-bit resolution which is equivalent to 28 or 256 positions per revolution. Since code disk 108 includes eight tracks, detector 106 is usually implemented with a photodiode for each code track resulting in a total of eight photodiodes. Therefore, as shaft 110 rotates, the photodiodes of detector 106 receives different light signals that pass through code disk 108 causing light detector 106 to produce an on/off digital output signal that indicates the position of code disk 108.
However, there are disadvantages associated with conventional absolute encoders. For example, if you want to increase the resolution of a code disk (e.g., 108), the number of code disk tracks are usually increased thereby resulting in an increase in the diameter of the code disk. Additionally, as the number of code disk tracks increases, the number of photodiodes utilized within a light detector (e.g., 106) are usually also increased. As a result of these modifications, the cost and size of the absolute encoder system undesirably increases.